back by using an attrgetter for the default case

This supports the use case of an application that uses the same set of
:class:`.Table` objects in many schemas, such as schema-per-user.
A new execution option
:paramref:`.Connection.execution_options.schema_translate_map` is
added. fixes #2685
- latest tox doesn't like the {posargs} in the profile rerunner

attempt integer indexed access from a key/value object nor
string access from an integer-indexed array, as earlier Postgresql
versions (prior to 9.4) don't allow this

persistence of JSON values in MySQL as well as basic operator support
of "getitem" and "getpath", making use of the ``JSON_EXTRACT``
function in order to refer to individual paths in a JSON structure.
fixes #3547
- Added a new type to core :class:`.types.JSON`.  This is the
base of the PostgreSQL :class:`.postgresql.JSON` type as well as that
of the new :class:`.mysql.JSON` type, so that a PG/MySQL-agnostic
JSON column may be used.  The type features basic index and path
searching support.
fixes #3619
- reorganization of migration docs etc. to try to refer both to
the fixes to JSON that helps Postgresql while at the same time
indicating these are new features of the new base JSON type.
- a rework of the Array/Indexable system some more, moving things
that are specific to Array out of Indexable.
- new operators for JSON indexing added to core so that these can
be compiled by the PG and MySQL dialects individually
- rename sqltypes.Array to sqltypes.ARRAY - as there is no generic
Array implementation, this is an uppercase type for now, consistent
with the new sqltypes.JSON type that is also not a generic implementation.
There may need to be some convention change to handle the case of
datatypes that aren't generic, rely upon DB-native implementations,
but aren't necessarily all named the same thing.

insert-default holding columns not otherwise included in the SET
clause (such as primary key cols) to get rendered into the RETURNING
even though this is an UPDATE.

- Major fixes to the :paramref:`.Mapper.eager_defaults` flag, this
flag would not be honored correctly in the case that multiple
UPDATE statements were to be emitted, either as part of a flush
or a bulk update operation.  Additionally, RETURNING
would be emitted unnecessarily within update statements.

fixes #3609

cause a subsequent call to the :meth:`.Query.with_parent` method to
fail. fixes #3606
- add mark-as-fail test for #3607

scope of a :meth:`.Session.flush` operation that's raising an
exception, as has been observed in some MySQL SAVEPOINT cases, prevents
the original  database exception from being observed when it was
emitted during  flush, but only on Py2K because Py2K does not support
exception  chaining; on Py3K the originating exception is chained.  As
a workaround, a warning is emitted in this specific case showing at
least the string message of the original database error before we
proceed to raise  the rollback-originating exception.
fixes #2696